Consolidated Statements Of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Statement [Abstract]
Revenues	$ 766,332	$ 965,685	$ 1,503,702	$ 1,925,922
Costs and Expenses [Abstract]
Cost of sales (exclusive of items shown separately below)	656,198	799,558	1,305,941	1,605,817
Depreciation, depletion and amortization	111,085	124,536	221,278	255,689
Amortization of acquired sales contracts, net	(2,209)	(4,451)	(5,019)	(18,468)
Change in fair value of coal derivatives and coal trading activities, net	(9,008)	(32,054)	(7,700)	(35,667)
Asset impairment and mine closure costs	20,482	525,583	20,482	525,583
Goodwill impairment		115,791		115,791
Selling, general and administrative expenses	34,302	35,178	67,511	66,039
Other operating income, net	(8,239)	(1,563)	(11,081)	(19,766)
Total operating expenses	802,611	1,562,578	1,591,412	2,495,018
Loss from operations	(36,279)	(596,893)	(87,710)	(569,096)
Interest expense, net:
Interest expense	(94,756)	(78,728)	(189,830)	(153,500)
Interest and investment income	1,216	1,088	4,052	2,109
Interest expense, net:	(93,540)	(77,640)	(185,778)	(151,391)
Other nonoperating expense:
Net loss resulting from early retirement and refinancing of debt		(19,042)		(19,042)
Loss from continuing operations before income taxes	(129,819)	(693,575)	(273,488)	(739,529)
Benefit from income taxes	(49,468)	(251,119)	(108,821)	(282,974)
Loss from continuing operations	(80,351)	(442,456)	(164,667)	(456,555)
Income from discontinued operations, net of tax	8,145	7,032	22,412	22,540
Net loss	(72,206)	(435,424)	(142,255)	(434,015)
Less: Net income attributable to noncontrolling interest		(65)		(268)
Net loss attributable to Arch Coal, Inc.	$ (72,206)	$ (435,489)	$ (142,255)	$ (434,283)
Loss per common share
Income (Loss) from Continuing Operations, Per Basic Share	$ (0.38)	$ (2.09)	$ (0.78)	$ (2.15)
Income (Loss) from Continuing Operations, Per Diluted Share	$ (0.38)	$ (2.09)	$ (0.78)	$ (2.15)
Basic earnings (loss) per common share	$ (0.34)	$ (2.05)	$ (0.67)	$ (2.05)
Diluted earnings (loss) per common share	$ (0.34)	$ (2.05)	$ (0.67)	$ (2.05)
Weighted average shares outstanding
Basic, weighted average shares outstanding	212,082	212,048	212,072	211,868
Diluted, weighted average shares outstanding	212,082	212,048	212,072	211,868
Dividends declared per common share	$ 0.03	$ 0.03	$ 0.06	$ 0.14